This invention relates to a nuclear magnetic resonance apparatus for measuring the concentration distribution or the like of a specified atomic nucleus in a body to be examined from the outside of the body without any surgical invasion by the use of the nuclear magnetic resonance (NMR) phenomenon.
The general construction of such diagnostic NMR apparatus is shown in FIG. 1. In this construction, projection data from plural directions are obtained without any rotation or parallel movement of the body being examined. This is accomplished by ascertaining the distribution of a specified atomic nucleus in a predetermined angular direction with the use of a gradient magnetic field overlapping a static magnetic field and by electrically rotating the gradient direction of the gradient magnetic field with respect to a specified plane of the body.
In FIG. 1, an oscillator 1 generates high-frequency waves the amplitude of which can be changed and a bridge type receiver 2 detects an NMR signal from a body P to be examined by use of the output of the oscillator 1. A coil 3 is wound around the body P for impressing the high-frequency waves (e.g., electromagnetic waves) from the oscillator 1 through the receiver 2 upon the body P and for extracting the NMR signal from the body P and feeding the signal back to the receiver 2.
An amplifier 4 amplifies the NMR signal fed to the receiver 2 and a recorder 5 records the NMR signal in synchronism with modulation of a magnetic field which is established by later-described modulating coils 12. A reconstructor 6 reconstructs the concentration distribution image of the specified atomic nucleus (e.g., hydrogen) from the NMR projection signals which are recorded in the respective angular directions by the recorder 5. A display 7 displays the image which is reconstructed by the reconstructor 6.
A pair of electromagnets 8 generates a homogeneous static magnetic field between magnet plate members 10 of the electromagnets and a D.C. stabilizing power supply 9 drives the electromagnets. A pair of shims 11 made of a magnetic materials and protruding from opposing surfaces of the magnet plate members 10 establish a magnetic field that is gradient with respect to the shown direction Y. Modulating coils 12 are driven by an A.C. power supply 13 and establish an alternating magnetic field of low frequency in a manner to overlap the static magnetic field.
Gradient magnetic field coils 14 are driven by a control power supply 15 (e.g., a stabilizing power supply) and establish such a magnetic field as is gradient with respect to the shown direction Z, acting as a gradient magnetic field electromagnet device. By controlling the output of the control power supply 15, it is possible to vary the gradient of the magnetic field which is established by the gradient magnetic field coils 14 and which is gradient with respect to the direction Z. Since the total gradient magnetic field can be established as the vector summation of the gradient magnetic field in the direction Z and the gradient magnetic field in the direction Y, the total gradient magnetic field can be rotated by controlling the output of the control power supply 15.
With the construction thus far described, the projection data of the concentration of the specified atomic nucleus in a specified plane of the body in various angular directions can be obtained without any rotation of the body P.
However, the method thus far described has the following problems:
(a) If a static magnetic field (Ho) varies with the drift of the stabilizing power supply 9 and/or the variation, or the like, in the spacing between the magnet planes due to the variation in the room temperature, the projection information signal PS, is shifted, as shown in FIG. 2, in accordance with those variations with respect to the phase of the output current of the A.C. power supply 13 for the modulating coils, as indicated at PS2. In other words, the body P appears to be moved. If this variation proceeds during the collecting procedure of the projection data, a distortion is established in the reconstructed display image.
(b) If the gradient of the gradient magnetic field (Gz) is varied as a result of the drift or the like of the control power supply 15 for the gradient magnetic field coils 14 similar to the case (a), the projection information signal PS3 is either compressed (in case the gradient is increased), as indicated at PS4, or elongated (in case the gradient is decreased), as indicated at PS5, in a manner to correspond to the variation in the gradient of the gradient magnetic field (Gz), as shown in FIGS. 3(a), (b) and (c), respectively. Since, in this case, the position of the body P is also apparently varied, the reconstructed picture image is distorted if the variation takes place during the collecting procedure of the projection information signal. Incidentally, in case the static magnetic field (Ho) has an intensity of 1 to 10 K Gauss whereas the gradient magnetic field (Gz) has an intensity of 1 Gauss/cm, an apparent variation in the thickness of the body reaches as high as 5% even for the variation of 1/1000 to 1/10,000 because it corresponds to the fact that the body is varied 1 cm.
(c) Another but larger problem is that, since the high-frequency magnetic field (H1) to be established in the body P by the coil 3 is not uniform inside of the body P, the NMR signal from the specified atomic nucleus in the body P is projected with different weights according to the positional relationships with the coil 3. The distribution of the high-frequency magnetic field (H1) is highly distorted especially in the vicinity of the lead portion of the coil 3. It therefore follows that the projection information signal has to be compensated for each projection direction.